ARM: dts: meson6: use stable UART bindings

The UART bindings needs specifying a SoC family, use the meson6 family
for the UART nodes like the other nodes.
Switch to the stable UART bindings for meson6 by adding a XTAL node and
using the proper compatible strings.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>